HIV-1 invasion of the Central Nervous System, CNS, induces a variety of clinical abnormalities including dementia, also known as HIV-1 associated dementia, HAD. Histologically, brain biopsy from AIDS patients with neurological disorders exhibit a broad ranges of abnormalities, most notably neuronal cell loss. The absence of neuronal infection suggested an indirect role for the virus to induce neuronal cell damages. Results from a large number of studies indicated that expression and secretion of several cellular proteins including cytokines and immunomodulators from the infected macrophages, microglia, and astrocytes in the brain participate in the neuronal cell damages. In addition to cellular factors, the viral regulatory protein Tat has received attention due to its unique ability to be secreted by the infected cells, enter into uninfected cells and alter the expression of cellular genes in the absence of viral infection. Moreover, Tat protein has been shown to have neurotoxic activity, most likely through dysregulating signaling pathways that are critical for neuronal cell survival. By using PC12 neuronal cell model, as well as primary rat neuronal cells, we demonstrated that Tat stimulate the expression of Id gene, a known cellular protein that inhibits differentiation of neuronal cells. Id, by affecting expression and activity of several factors which control the cell cycle promotes unscheduled entering of quiescent cells into proliferating stage and that results in suppression of a group of proteins that are implicated in the maintenance of differentiated cells. The basic helix-loop-helix, bHLH, family of DNA binding proteins such as Ngn, NeuroD, and Mash1, are the primary targets for Id1 which upon dysregulation may promote neuronal cell injury and apoptosis. Our preliminary data suggest that activation of Id1 gene by Tat is mediated via a novel regulatory pathway that includes critical transcription factors such as Egr1, C/EBP and SP1, all of which physically and functionally interact with Tat. In Aim one of this project we will perform a series of comprehensive functional and structural studies to unravel the molecular mechanisms involved in activation of Id1 by Tat in neuronal cells. In Aim two, we will investigate the importance of Id1, whose expression is elevated by Tat, in neuronal cell cycle, control of expression and activity of factors controlling cell entry into cell cycle and checkpoints such as p21, p16, pRb, and various cyclins. Furthermore, the role of Tat-induced overproduction of Id1 on the expression and activity of bHLH will be assessed. The outcome of these studies will provide a new information on the pathways which are affected by HIV-1 Tat, and will assist to improve our effort for developing strategies against HIV-1 induced neuronal cell injury.